FUEL. . 175 early part of the process and use this in supplying cities with illuminating gas, reserving the later product, containing less illu-minants, for burning in the flues. The following remarks are quoted from Blauvelt:* "There are two distinct types of retort-ovens, viz., the vertical and horizontal flue types. In the former there are thirty-odd vertical flues in each wall between* the ovens. These are connected at the. top and bottom by larger horizontal flues, running the length of the oven, the lower one being divided into two parts by a partition midway between the ends. The gas is burned in the lower flue, the flame rising, through half the vertical flues and descending through the other half, and escaping usually to regenerators of the ordinary reversing type, which heat the air for the combustion. The course of the gases is reversed about every hour and sent through the flues in the opposite direction. | "In the horizontal flue oven the gas is burned in horizontal flues, usually three in number, which are connected at the ends to form a continuous system, the gas being admitted through small pipes at the ends of the top and middle flues, where it meets air for the combustion. The gases travel from above downward, pass under the bottom of the oven, through a recuperative arrangement for heating the air, and then to boilers, where steam is made for operating the plant.-" Fig. IX-rB is an example of the Semet-Solvay horizontal flue type at Ensley, Ala., while Fig. IX-C shows the regenerative Otto Hoffman ovens at the works of the Maryland Steel Company at Sparrow's Point, Md. Of the total number of coke ovens in the United States in 1903 as given in the Census Report, only about two per cent, were of retort construction, while in Germany there were not 2 per cent. of bee-hives. This difference is due to several causes. One is that the bee-hive oven makes a superior coke from Connellsville coal, and there is a prejudice or belief that the retort coke will not be as good. Another reason is that the cost of 'the ovens is very mucli greater. The advantages of retorts appear in using a coal poor in volatile matter, for when such coal is coked in bee-hives, a great deal of the fixed carbon must be burned to supply heat, and the yield of coke * Trans. A. I. Jf. J7., 1898. .